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Keef The three I'll concede are the bottom for sure, and quote easily staged. I think of the extraterrestrial, I'm not sure that any of them would be a reason to impose martial law, only on the basis that if a meteor hit and and did the kind of devastation that would enact martial law, we would probably be on our way to extinction. A "sun spot" that would be strong enough to take out our infrastructure, would certainly take out the militaries I'd argue that the best they would be lucky to manage would be short range comms. If, the radios and their signals weren't affected. Aliens though, would be a wild card, depending on their reason. However, I maintain on that subject that if they could travel that far, then they could just as easily wipe us out without us even knowing it was them. It sure as hell wouldn't be like Independence Day. So I believe any alien encounters we have on earth, will likely be of a neutral or peaceful nature.
That however, isn't exactly what I am getting at. My concern is actually a bit more sinister in nature. I am more concerned with an unwarranted martial law being enacted. That our government might stage some sort of coup against it's citizens. Unfortunately I can think of a number of reason why. Most in the plausible conspiracy range. Unfortunately there seem to be coincidences between these theories and the actions of our government.
P.S: as far as I know, FEMA isn't part of the US Armed Forces and don't have anything to do with martial law.
On the contrary, FEMA will be a very important and used when martial law is enacted.
http://www.training.fema.gov/emiweb/edu/docs/TopOff4_afteraction_report2007.pdf
In the above link, you can see that FEMA is very involved in the coordination of "assets."
Here is another article on TOPOFF 3, which seems well researched and everything has citation.
http://globalresearch.ca/articles/CHO504B.html
FEMA does a far amount of coordination for NEP as well, not to mention the new conspiracy, the FEMA death camps.
And then our fearless leader gives a lovely address that Rachel Maddow picked through well.
Considering the strange coincidences of "training for an attack" and actually having an attack occur, Such as the Boston Marathon bombing and the 7/7 London Subway bombing, I could easily see another TOPOFF becoming the actual staging of a large scale, multi nation martial law event.